1. Field of the Invention
The present invention relates to a process for removing waterborne flexographic printing inks from substrates containing them. More particularly, the invention comprises removing waterborne (hydrophilic inks) from a substrate by using pulping and a flotation deinking process wherein the aqueous mediums have a basic pH, and include a nonionic surfactant and an anionic polyelectrolyte and wherein the pulper includes an amount of paper stock, different from the stock being treated, which includes an inorganic filler or coating.
2. Technology Description
Waterborne inks used to print newspapers by flexographic processes offer reduced emissions of volatile organic compounds to the environment during the printing process compared to oil based ink formulations. These inks, however, interfere with the recycling of the same newspapers as the colloidal nature of the pigments used combined with their hydrophilic nature make their separation from paper fiber difficult. The use of these inks can be viewed as an exchange of one type of environmental liability for another.
Waterborne or hydrophilic inks are characterized as having water as the vehicle or major liquid component in which is dissolved or suspended the large number of additives used by the ink formulator to provide a variety of desirable properties required for application on the printing press. These additives may include pigments and dyes (e.g., carbon black), for coloring the printed surface, binders such as dissolved or suspended polymers (e.g., polyacrylate), thickeners to control the rheology of the ink, water miscible solvents such as alcohols to control drying rate, surface tension, settling stability and other properties. In addition surfactants to control a variety of surface properties and control or minimize foaming, and dispersants to maintain product quality in storage and use are common. Buffers and other pH modifiers are commonly used. One common class of waterborne inks contains a carboxylated polymer and a fugitive base such as ammonia hydroxide which is selected to provide a medium that the resin is soluble. The carboxylated polymer is soluble in the aqueous vehicle at high pH; when the ink is applied to paper on the printing press, the base vaporizes reducing the pH. This results in desolubilization of the polymer which improves the attachment of pigment to the substrate.
The impact of using waterborne inks on conventional deinking processes is severe; inclusion of as little as 5% flexographic news into a newsprint recycle furnish can significantly reduce the brightness of the recycled pulp.
Conventional practice has been to process flexographic newsprint using wash deinking processes. While wash deinking is easy to operate, and requires minimal capital investment, the large volumes of water required make wash deinking an increasingly environmentally unacceptable practice. The paper industry as a whole is under severe pressure to reduce rates of water consumption. As a result the trend in the recycled paper industry is towards use of flotation or hybrid flotation/wash systems for ink removal. Flotation is particularly ill-suited for removal of waterborne ink from recycle newsprint as both the small particle size and hydrophilic nature of the ink result in poor rates of bubble attachment and low separation efficiencies. The poor effectiveness of flotation in removal of waterborne inks is a particular obstacle to recycling newsprint so contaminated, and in some cases restricts the acceptance of the use of waterborne inks as a means to reduce emissions of volatile organic compounds.
There have been proposals describing a two stage process to remove waterborne pigments using a flotation step under acidic conditions followed by a flotation step under alkaline conditions. Maintenance of acid conditions limits the hydrophilicity of the pigment particles, and reduces the degree of dispersion of such inks. The benefit of this procedure is avoidance of the high water consumption of a wash system, however the capital and operating costs of this process are greater than that of a single flotation stage process, and in addition pulping under acidic conditions is difficult if the waste paper contains alkaline fillers such as calcium carbonate.
In Borchardt et al (TAPPI 1994 Pulping Conference, November 6-10, Proceedings pages 1067-1103) many of the difficulties of recycling flexographic newsprint are described in detail; in particular the extremely small size and hydrophilic nature of the pigment particles, and difficulty in removing them by either wash or flotation unit operations. Borchardt also discloses polyacrylates as effective in reducing redepositon of pigment in these applications.
In WO 93/21376 use of anionic polymers as aides in deinking wastepaper under substantially neutral conditions is described. This reference stresses that its process cannot be used at a pH of greater than 9 as yellowing of the pulped fibers can occur.
U.S. Pat. No. 5,094,716 describes the use of a combination of an anionic surfactant and an anionic dispersant for use in removal of hydrophobic inks in wash processes. This reference does not suggest the use of its process for treatment of hydrophilic inks or the use of a nonionic surfactant.
U.S. Pat. No. 4,599,190 describes use of polyelectrolyte dispersants in combination with nonionic surfactants in wash deinking of secondary fiber. This process suggested is not a flotation process and it is unclear from the teaching of the reference if it can be used to treat hydrophilic inks.
U.S. Pat. No. 4,347,095 describes the use of carboxylated polymers in the presence of alkaline solutions for reclamation of conventional waste paper. This reference does not suggest that its process can be used to treat hydrophilic flexographic inks; rather, it appears as its teachings solely relate to hydrophobic inks, which have completely different redeposition properties as compared to hydrophilic inks.
Despite the above teachings, there still exists a need in the art for a process for removing hydrophilic inks under alkaline conditions by using a flotation method as such a method provides an environmentally friendly, cost effective way to remove such hydrophilic inks.